1. Field of the Invention
The present invention relates to a piston for internal combustion engines, wherein a ceramic piston head member is integrally embedded in the top end portion of a piston main body consisting essentially of metallic material.
2. Description of the Related Art
With the recent progressive development in technology, particularly in the automobile industry and the like, there have been various proposals to devise an internal combustion engine which makes use of ceramic material as extensively as possible, including the provision of a piston with its piston head formed of ceramic material. Such a piston can be produced, for example, by placing a previously prepared ceramic piston head member in a die, and subsequently casting into the die a predetermined amount of molten iron, aluminum alloy, or other suitable metallic material to form a piston main body such that, upon solidification of the cast material, the piston head member forms an integral part of the piston.
Because of low heat conductivity or high heat insulating capacity of ceramic materials, the ceramic piston head member effectively suppresses the transmission of combustion heat to the base material of the piston main body, and thus serves to significantly improve thermal efficiency of the engine. On the other hand, however, when compared with metallic materials, ceramics have a relatively low thermal expansion coefficient, and this results in the metallic material during the solidification being subjected to a higher magnitude of shrinkage, often forming an axial clearance or play between the ceramic piston head member and the base material. With such a clearance or play, the ceramic piston head member cannot be firmly retained in the piston main body, and is liable to separate from the top end portion of the piston main body particularly during the operation of the engine accompanying a reciprocating motion of the piston.
In order to prevent formation of undesirable axial clearance or play between the ceramic piston head member and the piston main body during the casting of the piston, it is known to form an extension of a dovetail section on the lower surface of the ceramic piston head member protruding downwardly therefrom, to establish a form-locking connection between the two elements (refer, for example, to Japanese Utility Model Application Laid-open Publication No. 56-45,134).
When, however, the extension of the dovetail section is formed on the lower surface of the ceramic piston head member in the central region thereof to protrude also radially outwards, the extension is subjected to a severe shearing and/or bending stress during shrinkage of the base material forming the piston main body, and is very liable to be broken during the casting of the piston.
Mechanical failure of the extension of the dovetail section formed on the ceramic piston head member, which takes place during the casting of the piston, can be prevented more or less by arranging the extension on the lower surface of the piston head member around the peripheral region thereof to protrude also radially inwards. This is because the shrinkage of the base material forming the piston main body results in a sliding movement between the two elements along the side surface of the extension, allowing an undisturbed shrinkage of the base material of the piston main body. The piston head member so embedded in the top end portion of the piston main body is tightly retained in place with a predetermined retaining force. However, the above-mentioned arrangement of the piston head member encounters additional serious problems.
That is, when the piston is subsequently subjected to combustion heat during the operation of the engine, the heat is stored in the ceramic piston head member so that the temperature near the piston head approaches the melting point of the base material of the piston main body. Thus, the force for retaining the piston head member in the piston main body decreases considerably. The decrease of such a retaining force also takes place as an axial clearance or play is formed between the piston head member and the piston main body, during operation of the engine. With the decreased retaining force, the piston head member can no longer be retained in place in a stable and reliable manner.